Rough is good - friction and all that. Smooth is bad. Ever try to wipe your arse with that grease-proof paper stuff they put in public toilets? Well, suffice to say, you need to change your underwear and have a shower when you get home.

@Lusiphur
That's the most horrible picture I've seen in a while. Love it.

Anyway, that reminds me. Two mates of mine were eurorailing about 20 years ago, passing through Czechoslovakia, Hungary, Poland etc. One of them, Glyn, got caught short and needed the toilet. Luckily they were at a station, so off he went. Hywel, the other one, noticed that he'd been gone a while so went to investigate. There was Glyn stuck without any paper. Hywel, duly went to queue for a newspaper. After waiting an eternity, he reached the cashier, who promptly closed up shop and refused to serve him.
He went back to Glyn and explained the problem. Any scraps of paper was the answer (Glyn's stomach had not acclimatized to the local cuisine). Receipts and spent rail tickets were passed under the door, but they were not close to sufficient for the job at hand. The dirt bucket at his side was filling up slowly. Reluctantly, Hywel passed through the lowest Eastern European denominations. He started off with Hungarian, continued with Czech and to his disgust had to hand through his Polish Zlotys.
Of course the total amount was trivial considering the strength of the pound at the time, but would amount to a veritable fortune for a local. You just wonder what the cleaner would have done when he found the treasure chest. I try not to.